This invention relates to a burner for a gas-turbine combustion chamber, in particular for an aircraft gas turbine, which comprises a lean premix burner with centrally integrated stabilizing burner.
Lean premix burners for gas-turbine engines and for gas turbines in other applications whose combustion chambers burn a fuel-air mixture with high content of air at low combustion temperature and correspondingly reduced nitrogen oxide formation are generally known. The use of such burners is, however, disadvantageous in that the stability of the flame is not ensured. In other words, the air-fuel mixture supplied to the combustion chamber will not burn or be ignited continuously as the combustion temperature falls, as a result of which the flame will fluctuate or may even go out. On gas-turbine engines for aircraft, this problem exists, in particular, at low ambient temperatures, in hail or rain showers or under similar, adverse meteorological conditions resulting in a reduced temperature of the air-fuel mixture. For ignition of the air-fuel mixture, a sufficiently high air temperature is required to rapidly vaporize the liquid fuel supplied to the combustion chamber as droplet mist, preheat it to a temperature as high as possible, depending on the composition of the fuel-air mixture and, thus, facilitate ignition.
In order to ensure ignition of the air-fuel mixture at any time, an ignition or stabilizing burner is, as is generally known, allocated to the lean premix burners arranged in the combustion chamber which produces a high combustion temperature with an air-fuel mixture with higher fuel content (rich mixture) to enable ignition of the air-fuel mixture supplied by the lean premix burner or main burner, which due to its weakness delivers a low combustion temperature, even at low air temperatures and correspondingly unfavorable vaporization behavior of the liquid fuel and to ensure the stability of the flame.
Normally, combustion chambers including lean premix burners with stabilizing means are of the staged design, with a stabilizing burner being allocated separately to each main/lean premix burner in a laterally staged arrangement. Besides complexity, high number of parts, high manufacturing costs and high weight, cooling of the large surfaces involves considerable investment. These combustion chamber concepts are generally known as “axially staged combustion chambers” or “dual annular combustion chambers”.
Other types of lean premix burners using stabilizing means in which the ignition burner is centrally integrated do not have the design disadvantages described above, but are not considered successful since they fail to satisfy both a lean overall ratio of the air-fuel mixture required and stable operation of the centrally arranged stabilizing burner. Particularly critical here are idle operation of the gas turbine where the air entry temperature to the combustion chamber is particularly low and run-up of the gas turbine upon engine start when in part very high total air-fuel mixture ratios are to be handled. Besides this, transient operating points must be flyable: Particularly unfavorable here is the transition from part load in cruise to flight idle in descent.
Further, maneuvers are encountered in which engine thrust must be reduced very rapidly, with the decrease in fuel flow leading to extremely weak air-fuel ratios. In addition, all these unfavorable operating points must, as already mentioned, be flyable under extreme meteorological conditions, such as hailstorms or tropical rain. Furthermore, such conditions must be manageable as they are encountered during re-start of the engine or re-light of the combustion chamber at elevated altitudes, i.e. under atmospheric conditions with very low pressure and low temperature (up to minus 56° C.).
A burner combination of the type mentioned above, which comprises a main burner with centrally integrated stabilizing burner, is described in Specification EP 0 660 038 B1, for example. This burner comprises a main burner with an annular, external fuel-air mixing duct for the production of a fuel-air mixture to be supplied to the combustion chamber and a stabilizing burner provided in an axial duct of a central body, i.e. centrally located in the main burner, at whose exit port fuel is sprayed and is introduced, mixed with core air, into the gas-turbine combustion chamber. A flame formation which is stable throughout the range of operating conditions can, however, not be achieved With this burner design.